The present invention relates generally to stainless steel alloys and, in particular, to stainless steel alloys having well balanced physical properties, such as good corrosion resistance, intermediate strength and good machinability, and further being easily processed and having a low preparation cost.
A variety of stainless steels are currently utilized in commerce, primarily for applications requiring the strength of steel, but also having excellent corrosion resistance to perform the desired function without degradation of structure and appearance due to a corrosive environment. There are five basic categories of stainless steels; martensitic, ferritic, austenitic, maraging and semi-austenitic. The physical properties of these various categories of stainless steel can be characterized with regard to corrosion resistance, strength, machinability, ease of processing and cost to prepare stock.
Martensitic stainless steels consist essentially of 12-14% by weight of Cr plus other elements to enhance selected specific properties. Hereinafter, elemental percentages are expressed in weight percentages unless otherwise specified. The martensitic stainless steels have the poorest corrosion resistance of all the stainless steels. Martensitic stainless steels can be heat treated to wide strength levels, from low strength to very high strength. The straight Cr alloys, such as a UNS S41000, have good machinability in the annealed condition, or when heat treated to an intermediate strength level. The machinability can be significantly improved by increasing additions of sulfur, such as in a UNS S41600 alloy. The martensitic stainless steels are relatively easy to hot-work, but they must be annealed after the hot working operation. The alloys which contain only Cr and a medium C level are relatively easy to anneal, and the annealing factors are not critical. Because improper processing can result in high hardness and stress cracking, the grades of stainless steel which contain high C and/or Ni require close control of the following process parameters: cooling rate after hot working treatments, annealing temperatures and cooling from the annealing temperature. In the annealed condition the martensitic stainless steels cold-work ather easily and can also be cold worked at a relatively low, intermediate strength level. These martensitic stainless steel alloys are typically heat treated in a two step process. The first step is to austenitize at a high temperature (usually 1700.degree. F. to 2000.degree. F.) and then oil or air quenched to room temperature. In the second step the material is tempered or drawn to the desired strength level at a relatively low temperature (800.degree. F. to 1400.degree. F.). In most situations the tempering time and temperature are quite critical parameters. Generally the martensitic stainless steels tend to be the lowest cost of all of the stainless steels.
Ferritic stainless steels contain about 16-30% Cr and can contain other elements, such as Mo to enhance specific properties. A UNS S43000 stainless ssteel containing 17% Cr has good machinability, and the machinability can be significantly improved with increasing additions of S. The ferritic stainless steels have low strength, and the strength cannot be increased by heat treating. The ferritic stainless steels also have a low rate of work hardening, and the strength cannot be significantly increased by cold working. Ferritic stainless is relatively easy to process in regard to hot working, cold working and annealing. Stainless steels such as the above mentioned US 43000, which are air melted alloys, are relatively inexpensive to produce, but the higher alloy vacuum melted stainless steels, such as UNS S44625, are relatively expensive to produce.
Austentic stainless steels are best exemplified by a UNS S 30400 alloy which is commonly referred to in the art as "18-8". In the austenitic Cr-Ni steels the Cr and Ni contents can vary substantially to meet various applications, and other elements, such as Mn, Si, Mo, Cu, Ti, Cb, and N, can be added to enhance selected properties. These alloys are best known for their excellent corrosion resistance properties. The hot worked or cold worked 18-8 alloy is annealed at about 1800.degree. F.-2000.degree. F., followed by a rapid cooling, typically by water quenching. The austenitic stainless steels cannot be hardened by heat treating, but the strength can be increased by cold working because these alloys have a high rate of work hardening. The alloys are relatively easy to process since they have good hot and cold workability and require only a simple annealing treatment. The austenitic stainless steels are usually higher in cost than the martensitic and ferritic stainless alloys because the necessary raw materials include more expensive higher alloy starting material. The machinability is fair to good and can generally be improved with the addition of S.
The maraging stainless steels are martensitic alloys which have very low carbon plus nitrogen content and can be precipitation hardened. In general these alloys have low percentages of ferrite and retained austenite. Examples of maraging stainless steels are UNS S17400 and UNS S36200 alloys which have good corrosion resistance. In an as solution annealed state these alloys have intermediate strength, and they can be precipitation hardened from an intermediate to a high strength level. The machinability is however rated as being poor, and the maraging steels are not among the easiest alloys to process. The maraging stainless steels also must be melted under carefully controlled chemistry conditions in order to prevent hot working problems and to insure proper response to heat treatment. The cold workability will depend on the alloy and the heat treatment, but this category of steels is usually rated as having fair to poor cold workability. In general there are three basis heat treatments for the maraging alloys: (1) a single or double overage to place the material in condition for optimum cold working or machinability, (2) a solution anneal which is required prior to a precipitation hardening cycle, and (3) a precipitation hardening to a specified strength leve. These alloys are not rated as low cost alloys due in part to the lengthy processing schedule.
The semi-austenitic stainless steels are austenitic in the solution annealed condition, and some alloys of this category may contain up to 20% to 30% ferrite, as solution annealed. By means of several heat treating operations the austentite can be transformed to martensite, and the martensite is usually age hardenable. The chemical composition of steels of this category should be closely controlled in order to produce austenite of the proper stability so that the martensitic transformation will occur. Strength levels from intermediate to high strength are produceable by heat treating. High strength also is produceable by cold working which transforms the austenite to martensite, followed by aging or tempering treatments. Examples of semi-austenitic stainless steels are UNS S17700, UNS S35500 and UNS S15700 alloys. These alloys have good corrosion resistance, but their machinability is rated as poor. The semi-austenitic stainless steels are not easily produced and are not low cost alloys.
Each of the above categories of stainless steels offer various advantages and features, but each generally lacks certain significant properties and none has a well balanced set of physical parameters attainable by using materials of modest cost and straightforward processing methods.